1. Field of the Invention
The invention relates to a piston for internal combustion engines, particularly pistons having crown plates.
2. Description of the Prior Art
When such pistons are heated to working temperature, the crown domes upwards around the axial retaining screws. For the protection of the axially extending necked-down screws against bending stresses which easily lead to breakage, by reason of the doming of the crown, it was already proposed in German Publ. Spec. No. 12 69 414 to provide a radially outwardly widening gap between the abutment surfaces of piston crown and piston body which are braced against one another by the necked-down screws. The effect of this gap in relation to the screw loading is described in detail in Ger. Pub. Sp. No. 12 69 414.
Now the upward doming of the piston crown influences not only the abutment faces connected with one another by the necked-down screws, but also the radial annular fit between piston crown and piston body, serving for the centring of the two parts. As a result of the upward doming of the piston crown the annular surface of the piston crown also seeks to place itself obliquely in the annular fit between crown and piston body. Thus the zone of the annular fit lying towards the piston foot is more heavily loaded than the zone of the annular fit pointing towards the piston head. The oblique position can proceed so far that an annulr gap widening towards the piston head develops in the annular fit at working temperature. The unequal abutment of the two surfaces of piston crown and piston body which meet in the annular fit leads, as shown by practice, to the development of friction rust especially in the zone of the annular fit facing towards the piston foot.
This is explained in that micro-displacements occur between the adjoining surfaces in the annular fit under pressure which is elevated in comparison with room temperature specifically in the zone towards the piston foot. More especially the elevated pressure in the lower fit zone appears to determine the development of the occurring friction rust. It is however also possible that the micro-displacements are rendered possible only by the gap formation at working temperature in the annular fit.